Breathing is significantly altered during the loss of wakefulness. The sleep apnea syndromes are currently estimated to affect 1 to 5% of the adult male population. About 7,000 children in the U.S. die every year from sudden infant death syndrome (SIDS). Respiratory pauses, (apneas) during sleep are common to both adult sleep apnea and SIDS. In affected individuals, breathing is particularly disrupted during rapid eye movement (REM) sleep. The neuronal and neurotransmitter mechanisms which cause respiratory depression during REM sleep are poorly understood. Studies in the last three years have shown that cholinergic mechanisms in the pontine reticular formation, that are known to be involved in REM sleep generation, can also cause state-dependent respiratory depression. Encouraged by these findings, the long-term objectives of this continuing application are to characterize the role of endogenous cholinergic neurotransmission in the medial pontine reticular formation as a contributor to state-dependent respiratory depression. The proposed studies will all use intact, unanesthetized animals studied during wakefulness, NREM sleep, REM sleep, and during the cholinergically-induced REM sleep like state. This proposal for 5 years of respiratory neurobiology research has 2 specific aims: (1) Using brain microdialysis, high performance liquid chromatography, and electrochemical detection, Aim 1 will test the hypothesis that increased acetylcholine (ACh) release within the pontine reticular formation contributes to state-dependent respiratory depression. (2) While the pontine reticular formation contains cholinergic receptors (i.e., it is cholinoceptive), this brain region does not produce ACh (i.e., it is not cholinergic). All ACh release in the medial pontine reticular formation arises from the laterodorsal and pedunculopontine (PPT) tegmental nuclei. Accordingly, Aim 2 will test the hypothesis that the PPT nuclei contribute to state-dependent changes in breathing via cholinergic projections to the medial pontine reticular formation. This hypothesis will be examined by electrically stimulating the PPT while quantifying respiration and measuring ACh release in the medial pontine reticular formation.